Bisbenzotriazoleureas as UV stabilizers for polymers

ABSTRACT

A hydroxyphenyl-benzotriazole bisurea UV light stabilizer for organic polymers and a process for making it are provided. The stabilizer is particularly useful for spandex polymer yarns that are subjected to solvent scouring. A preferred stabilizer of the invention is a bisurea formed by reacting methylene-bis(4-phenylisocyanate) with 2-[2-hydroxy-3-(1,1-dimethylethyl)-5-(N-aminoethyl)propionamid-phenyl]-2H-benzotriazole.

RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 07/196,591, filedMay 19, 1981 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to chemical compounds which are used asstabilizers for protecting polymers against degradation that is inducedby exposure to ultraviolet light. Specifically, the invention concernssuch a UV-stabilizer, which is a bisurea compound that includes two2-(2'-hydroxyphenyl)benzotriazole groups.

2. Description of the Prior Art

Many compounds are known in the art for use as UV-stabilizers fororganic polymers. Among the known UV-stabilizers are2-(2'-hydroxyphenyl)benzotriazoles of the type disclosed by CanadianPatent 1,197,246 to Ciba-Geigy AG. Ciba-Geigy sells such UV-stabilizersunder the "Tinuvin" trade names. These include, for example, "Tinuvin"213, also referred to as "Tinuvin" 1130, which is mainly the2-[2-(2-hydroxy-ethoxy)ethoxy]ethylester of3-(2H-benzotriazol-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)-benzenepropanoic acid).

Although the 2-(2'-hydroxyphenyl)benzotriazoles have been useful in manyorganic polymers, when used as UV-stabilizing additives in spandexfibers certain shortcomings are evident. For example, spandex fibersoften are converted into fabrics or garments that are subjected tosolvent scouring and various dry-cleaning solvents. Solvents, such asperchloroethylene and other non-polar solvents, are used extensively.However, such solvents extract the hydroxy-benzotriazole additives fromthe polymeric fibers, thereby reducing the UV-protection for the fibers.Also, some of the known hydroxyphenyl-benzotriazole UV-light stabilizershave caused accelerated degradation of spandex in the presence ofaqueous hypochlorite solutions. Such solutions are commonly used aslaundry bleaches. Furthermore, some known hydroxyphenyl-benzotriazolestabilizers have a tendency to form colored complexes when exposed tocopper ions. In spandex, these stabilizers can cause yellowdiscoloration during textile finishing or in laundering, if the waterused contains copper ions (e.g., as might be present in water suppliedthrough copper pipes).

It is a purpose of this invention to overcome, or at least significantlyreduce, the above-recited shortcomings of the knownhydroxyphenyl-benzotriazole UV-stabilizers and to provide aUV-stabilizer that is not readily extracted from polymer by scouring anddry-cleaning solvents, is relatively unaffected by common laundrybleaches and is not detrimentally affected by the presence of copperions.

Although not related to 2-(2'-hydroxyphenyl)benzotriazole UV-lightstabilizers, bisurea compounds per se, as well as for other types ofstabilizers, are known in the art, as for example from Japanese Patent46/13143 to Teijin Ltd.

SUMMARY OF THE INVENTION

The present invention provides an ultraviolet-light stabilizer fororganic polymers, the stabilizer being a bisurea compound of thestructural formula (I) ##STR1## wherein

R denotes a divalent group which connects the two isocyanate groups ofan organic diisocyanate, preferably bis(p-phenylene)methane,bis(4-cyclohexylene)methane or 3,3,5-trimethyl-5-methylene-cyclohexyl,

R¹ is a saturated hydrocarbon chain of 2 or 3 carbon atoms and

R² denotes a monovalent radical containing a2-(2,'-hydroxyphenyl)benzotriazole group, preferably,3-(2H-benzotriazol-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)benzenepropanoyl.A particularly preferred UV-stabilizer of the invention has thestructural formula (II) ##STR2## where R is bis(p-phenylene)methane andR² is3-(2H-benzotriazol-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)benzenepropanoyl.

The present invention also provides (a) a process for preparing theUV-light stabilizer, (b) a new compound that is useful as anintermediate for preparing the UV-stabilizer, (c) a polymer containingan effective amount of the stabilizer and (d) a spandex yarn containingthe stabilizer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The 2-(2'-hydroxyphenyl)benzotriazole bisurea compounds of the inventionare suitable for use as UV-light stabilizers for organic polymers,including polyolefins, polyurethanes, polyamides, polyaramides,elastomeric polyurethanes, and the like. The compounds of the inventionare believed to be effective UV-light stabilizers for any polymer inwhich current commercial 2-(2'-hydroxyphenyl)benzotriazole UV-lightstabilizers have been used satisfactorily. However, the bisureacompounds of the invention are most useful in segmented elastomers,especially spandex polymers which have polyether soft segments.

In the 2-(2'-hydroxyphenyl)benzotriazole bisurea UV-stabilizers of theinvention, the hydroxyl group is located on the phenyl ring in the orthoposition with respect to the benzotriazole group. In addition, thephenyl group optionally can have other substituents, such as linear orbranched alkyl or arylalkyl groups. The benzotriazole group may alsocontain substituents such as halogen or alkyl groups. Such substituentsare believed to protect the phenolic group, to prevent harmfulsubstitutions in the ortho or para positions, or to modify theUV-absorption spectrum of the stabilizer. In addition, branched alkylgroups with no more than eight carbon atoms or α-substituted benzylgroups in the ortho position to the phenolic hydroxyl group are believedto increase the effectiveness of hydroxyphenylbenzotriazole stabilizersby affecting the steric position of the hydroxyl group versus thetriazole ring atoms. The UV protection provided by thehydroxyphenyl-benzotriazole light stabilizers is believed to be due totheir screening action, which results from their strong absorption ofultraviolet light.

To function effectively as a UV-light stabilizer in an organic polymer,the stabilizer of the invention usually is present in a concentration ofat least 0.1%, and seldom greater than 5%, based on the weight of thepolymer. For spandex polymer, the preferred concentration range is 0.5%to 1.5%. In general, spandex compositions with polyether soft segmentsneed somewhat higher stabilizer concentrations than those with polyestersoft segments. In other polymers, a different optimum concentrationrange may apply.

The effectiveness of the UV-light stabilizers of the invention can beenhanced by adding other stabilizers, such as phenolic antioxidants,hindered amines and/or phosphites, to the polymer. Additives, that areradical scavengers are especially beneficial. Such radical scavengers(e.g., phenolic antioxidants) prevent degradation which can result fromradical chain reactions which may have been initiated in spite of thepresence of the hydroxyphenyl-benzotriazole.

The UV-stabilizers of the invention can be incorporated into polymers byconventional techniques, much in the same way as other stabilizers areincorporated. For solution-spun spandex, the preferred way is to mix thestabilizer with other additives and a dilute polymer solution to form aslurry which can be mixed with polymer solution for spinning, asdescribed in Example 2, below. The compounds of the invention aresoluble in commonly used spandex-spinning solvents, such asdimethylformamide and dimethylacetamide, which facilitates theirincorporation into the spinning solutions and avoids formation ofparticles which may plug the spinnerets.

The process of the present invention comprises forming a2-(2'-hydroxyphenyl)benzotriazole derivative (referred to herein as anintermediate product) that has a substituent which includes a groupcapable of reacting with an organic diisocyanate, and then reacting theintermediate product with an organic diisocyanate to form the desired2-(2'-hydroxyphenyl)benzotriazole bisurea UV-stabilizer of theinvention. Preferably, the diisocyanate is selected from the groupconsisting of p,p'-methylene diphenyl diisocyanate,4,4'-methylene-bis(cyclohexylisocyanate) and isophorone diisocyanate. Ina preferred process, the methyl ester of3-(2H-benzotriazole-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)benzenepropanoicacid is reacted with excess ethylene diamine to form2-[3-(2H-benzotriazole-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)-benzenepropanoyl]amido-ethyl amine, which is then reacted with MDI to form ahydroxyphenyl-benzotriazole bisurea UV-stabilizer of the invention.Instead of the methyl ester of3-(2H-benzotriazole-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)benzenepropanoicacid, other esters of this acid can be employed as the startingmaterial.

Commercially available 2-(2'-hydroxyphenyl)benzotriazole lightstabilizers sometimes contain ester groups. Such benzotriazoles (e.g.,"Tinuvin" 213) are particularly suited for preparation of bisureasaccording to the invention. The ester derivatives of benzotriazoles canbe synthesized by known methods of organic chemistry and then convertedinto amide-amines by reaction with excess of a diamine to yieldintermediates suitable for preparation of bisurea stabilizers accordingto the invention. However, it is not necessary to start with esters, orto use amide-amine derivatives, to synthesize compounds of the abovegeneral structural formula.

Other starting materials useful for preparing bisurea stabilizers of theinvention include2-(2'-hydroxy-3-chloracetamidomethyl-5-methyl-phenyl)-2H-benzotriazoleand3-[3-(2H-benzotriazol-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)-phenyl]-propylchloride. A Gabriel reaction, for example, can be used to convert thesestarting materials into an intermediate which contains an amine groupthat is capable of reacting with diisocyanates to form the desired2-(2'-hydroxyphenyl)benzotriazole UV-light stabilizer.

In the above-described processes, the intermediate that is formed hasthe following generalized structural formula (III): ##STR3## where R islower alkyl or halogen, either the X or Y group is --R'--NH2 in which R'is a linking moiety optionally containing amide linkages, and the otherX or Y group is substituted or unsubstituted alkyl or aralkyl. Theseintermediates are most convenient for conversion into the bisureas ofthe invention by reactions with the preferred organic diisocyanates,methylene-bis(4-phenylisocyanate),4,4'-methylene-bis(cyclohexylisocyanate) or isophorone diisocyanate.

In the Examples which follow, several tests are employed to thedetermine the effectiveness of UV-light stabilizers in preventingdegradation and discoloration of yarns of spandex polymer.

Accelerated UV-light exposure tests are performed in an AtlasWeatherometer. In these tests, spandex fibers are exposed whilestretched, to accelerate any degradation. Degradation is more rapidbecause of the smaller diameter of the stretched fiber and the fibersexhibit irreversible "growth" in the early stages of degradation. Ineach test, a set of ten spandex threads, held just taut (i.e.,straight), is clamped in a metal rack with 10 cm between the clamps. Theclamps are then moved apart to a distance of 15 cm, stretching thethreads 50%. The stretched threads are then exposed for 20-hour periodsin the weatherometer. After each exposure period, the samples arerelaxed for 15 minutes by moving the clamps together until the strandsare slack. The final length of the threads, L, is then measured with thethreads held just taut enough to straighten the threads. The growth ofthe thread is expressed as a percent of amount of stretch in the threadwhile under test and is calculated by the formula

    % growth=(100)(L-10)/(15-10)=20(L-10).

Discoloration from UV-exposure, usually yellowing, is measured by anincrease in "b" value with a "Colorquest" colorimeter sold by HunterLaboratories. Yarns are wound on cards until the card is well covered.The yarn-covered cards are then subjected to light-exposure testing. The"b" values of the samples are recorded before and after exposure. Thechange in "b" value is a measure of the amount of yellowing.

The resistance of a UV-stabilizer to extraction from a polymericsubstrate by conventional scouring and dry-cleaning solvents is measuredby one of two tests; one for film samples and one for fiber samples.

For polymeric films, a 5-gram sample of film containing a lightstabilizer is immersed for 2 minutes in 0.1 liter of perchloroethylenesolvent which is stirred and maintained at 45° C. After removal from thesolvent and drying, the film is weighed. The weight loss of the filmsample is compared to the weight loss of a control sample that wasexposed to the same extraction treatment. The control sample is of thesame composition as the test sample but does not contain anyUV-stabilizer. The difference in percent weight loss between the testsample and the control sample represents the weight loss of UVstabilizer by the test sample and is expressed as a percentage of theoriginal amount of UV-stabilizer. The latter percentage is reported asthe "% extractability" of the UV-stabilizer.

For samples of polymeric fibers which contain several differentadditives, the following test is used. A 20-gram sample of yarn is cutinto pieces and then immersed and stirred for two minutes in 200milliliters of perchloroethylene maintained at 45° C. The fibers arethen filtered from the liquid. The perchloroethylene filtrate is vacuumevaporated to leave a first residue which includes any additives orfinishes that were removed from the sample by the perchloroethylene,along with any of the UV-stabilizer that was removed. To facilitateanalysis of the individual components of the residue, the residue isseparated into two fractions by extraction with hexane, ahexane-insoluble fraction and a hexane-soluble fraction. Evaporation ofthe hexane then results in a hexane-insoluble "Residue A" and ahexane-soluble "Residue B". Residues A and B are then analyzed by NMR(nuclear magnetic resonance) in deuterated chloroform to determine theamount of stabilizer in each of Residues A and B. The percentextractability of the UV-stabilizer is then calculated as 100 times thetotal amount of stabilizer in the combined residues divided by theamount present initially in the 20-gram yarn sample.

To measure the bleach-resistance of a spandex fiber, weighted samples offiber strands are suspended in an aqueous bath. The bath is maintainedat a pH of 7, a temperature of 28° C. and an active chlorineconcentration of 500 ppm. In the tests reported in Example I, a 5.8-gramweight is hung from a loop of 44-dtex spandex strand and the weightedstrand is placed in the bath. The exposure is continued until the strandbreaks. The duration of the test exposure is recorded. A UV-stabilizerthat does not accelerate chlorine-induced degradation, usually permitsspandex strands to endure for several hours in this test.

To determine the sensitivity of a spandex yarn containing a specificadditive to discoloration in the presence of copper ions, the followingtest is used. To 1 liter of water, 268.3 milligrams of copper chloridedihydrate are added to give a solution containing 10 ppm of copper. A1-gram sample of the yarn is soaked for four hours in 60 milliliters ofthe solution maintained at 45° C. The yarn is then removed from thesolution, washed with tap water and dried. The yellowness of the yarnsample is compared by eye to the yellowness of yarns containing otherstabilizers that were exposed to the same test conditions.

The examples which follow illustrate preferred embodiments of theinvention, but are not intended to limit its scope, which is defined bythe claims below. The following abbreviations and tradenames are used:

MDI: methylene-bis(4-phenylisocyanate).

PICM: 4,4'-methylene-bis(cyclohexylisocyanate).

IPDI: isophorone diisocyanate.

TDI: tolylene diisocyanate.

DMAc: N,N-dimethylacetamide.

Tinuvin 213: the 2-[2-(2-hydroxy-ethoxy)-ethoxy]ethylester of3-(2H-benzotriazol-2-yl)-4-hydroxy-5-(1,1-dimethylethyl)-benzenepropanoic acid), sold by Ciba-Geigy.

Cyanox 1790:1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)trione,an antioxidant sold by American Cyanamid.

DIPAM/DM: a copolymer of diisopropylaminoethyl methacrylate and n-decylmethacrylate in 75:25 weight ratio, a dye enhancer.

Tone 2000: a poly(caprolactone) diol of 2000 number average molecularweight, sold by Union Carbide.

Duponol EP: a detergent sold by E. I. du Pont de Nemours & Co.

Also, as used herein, the term "spandex" has its usual definition; thatis, a long chain synthetic polymer that comprises at least 85% by weightsegmented polyurethane.

EXAMPLE I

This example illustrates the preparation of a preferred2-(2'-hydroxyphenyl)benzotriazole bisurea UV-stabilizer compound of theinvention and its advantageous use in a spandex polymer yarn. Theperformance of the yarn is compared to spandex polymer yarns of the samecomposition but which (1) contain as UV-stabilizer, the2-(2'-hydroxyphenyl)-benzotriazole from which the preferred bisureacompound of the invention was prepared (Comparison Yarn A) and (2)contain no UV-stabilizer (Comparison Yarn B).

A solution of a hydroxyphenyl-benzotriazole bisurea stabilizer wasprepared by dissolving 454 grams of "Tinuvin" 213 UV-stabilizer in 4liters of methanol; adding 5 grams of p-toluenesulfonic acid; andheating the solution for 72 hours under reflux. The solution was thencooled to room temperature. A solid precipitate formed, which wasseparated from the liquid by filtration and then dried to form ayellowish powder that weighed 246 grams. The powder was the methyl esterof 3-(benzotriazol-2-yl)-4-hydroxy-5-t-butyl-benzene-propanoic acid. A210-gram portion of the methyl ester was dissolved in 1,800 grams ofethylene diamine and heated under reflux for 18 hours. Excess ethylenediamine and evolved methanol were removed by vacuum evaporation. Anorange residue remained. The residue was added to and stirred with 3liters of di-isopropyl ether. An amide-amine intermediate product wasprecipitated, which after filtration and drying, had a slightly yellowcolor and weighed 174 grams. The residue was dissolved in toluene andrecrystallized. The recrystallized residue, which melted at 146° C., was2-[2-hydroxy-3-(1,1-dimethylethyl)-5-(N-aminoethyl)-propionamid-phenyl]-2H-benzotriazole.This chemical is of the structure given by Formula III above, in which##STR4##

To a stirred solution of 128.8 grams of the thusly-obtained amide-amineintermediate product in 800 ml of DMAc, a solution of 42.3 grams of MDIin 200 ml of DMAc was added. During the addition, the reaction mixturewas cooled in ice to maintain the temperature below 40° C. The resultingsolution contained a bisurea compound of the invention having thefollowing structural formula: ##STR5##

A spandex yarn was prepared and stabilized with thehydroxyphenyl-benzotriazole bisurea UV-stabilizer described in thepreceding paragraph as follows. A solution was prepared of a segmentedpolyether-based polyurethane spandex polymer in DMAc by the generalprocedure described in U.S. Pat. No. 4,340,527, Example I. "Cyanox" 1790antioxidant, DIPAM/DM dye enhancer, Tone 2000 polyol and bisureaUV-stabilizer of the preceding paragraph were added to the solution inquantities that provided to the final spandex fibers additiveconcentrations of 1.5%, 2.0%. 0.5% and 1.0%, respectively, based on thefinal weight of the fiber (without finish).

The above-described spandex polymer solution was dry-spun in aconventional manner through orifices to form coalesced 4-filament,44-dtex yarns. Comparison Yarns "A" and "B" were spun from the samesolution, except that for Yarn A, "Tinuvin" 213 was substituted for theUV-stabilizer of the invention, and for Yarn B, no UV-light stabilizerwas included.

The above-described yarns were tested for growth on exposure toUV-light, extractability of the UV stabilizer by solvent anddiscoloration by copper ions. The results of the tests are summarized inthe following table.

                  TABLE I                                                         ______________________________________                                                   Of                                                                 Yarn Sample                                                                              Invention Comparison A                                                                              Comparison B                                 ______________________________________                                        Stabilizer Example I*                                                                              Tinuvin 213 None                                         Weight %   1.0       1.0         0                                            UV-exposure                                                                              17.5      21.3        30.8                                         20-hr growth, %                                                               Extractability,                                                                          12        45          --                                           % extracted                                                                   Copper ion test                                                                          light     yellow      clear                                        final color                                                                              yellow                                                             ______________________________________                                         *As described in the second paragraph of Ex. I.                          

The UV-growth data summarized in the table show that theUV-stabilization provided by the bisurea compound of the invention is atleast as good as, if not somewhat better than, that provided by Tinuvin213, at approximately the same concentration ofhydroxyphenyl-benzotriazole. After being subjected to solvent scouringand a subsequent 20-hour Weatherometer exposure test, the sample yarncontaining the bisurea compound of the invention still exhibited agrowth of only 23.1%. In contrast, the Comparison Yarn A, whichcontained "Tinuvin" 312, exhibited a 32.8% growth and the Coparison YarnB, which contained no UV-stabilizer at all, broke in testing.

The three yarns were also tested for degradation by chlorine laundrybleach. Average break times for the Comparison Yarn B (no UV-stabilizer)was 109 minutes. Comparison Yarn A (containing 1% "Tinuvin" 213)exhibited a break time of 52 minutes, indicating that this commercialUV-stabilizer accelerated chlorine-induced degradation. The yarncontaining the bisurea UV-stabilizer of the invention broke after 100minutes. Thus, in contrast to the commercial stabilizer, the bisureaUV-light stabilizer of the invention had only a very a small effect, ifany, on chlorine-induced degradation of the spandex fibers.

EXAMPLE II

Yarns were prepared as in Example I, except that 3.0% zinc oxide, basedon finish-free fiber weight, was added to the spinning solutions in theform of a slurry, substantially by the same procedures as described inU.S. Pat. No. 4,340,527. The yarns were wound on thin, rectangular metalplates and then subjected to a simulated finishing treatment. In thetreatment, the samples of wound yarn were immersed for 60 minutes inwater containing 0.3 gram per liter of ethylenediamine tetraacetic acidand 1.5 gram per liter each of tetrasodium pyrophosphate and "DuponolEP". The wound yarn samples were then exposed for 40 hours to UV-lightin the Weatherometer. The following increases in "b" values wereobserved to have resulted from the combined finishing and light-exposuretreatments:

                  TABLE II                                                        ______________________________________                                        Yarn Sample                                                                            Of Invention                                                                              Comparison A                                                                              Comparison B                                 ______________________________________                                        Stabilizer                                                                             Example I*  Tinuvin 213 None                                         Weight % 1.0         1.0         0                                            Increase in                                                                            8.1         8.4         11.3                                         "b" value                                                                     ______________________________________                                         *See Example I, Table I.                                                 

The data recorded in Table II again show that with regard to protectionagainst UV-light-induced discoloration, the2-(2'-hydroxyphenyl)benzotriazole bisurea UV-stabilizer of the inventionis as good as, if not somewhat better than, the commercial2-(2'-hydroxyphenyl)benzotriazole (i.e., "Tinuvin" 213) from which thestabilizer of the invention was made. However, as shown in Example I,the bisurea stabilizer compounds of the invention are considerablybetter in resistance to copper-ion-induced discoloration,chlorine-induced degradation and extractability of the stabilizer byscouring and/or dry-cleaning solvents.

Advantageous results, similar to those reported above, are obtained whenthis example is repeated with the R in Formula I being provided by PICMor IPDI, instead of by MDI as in this example and in Example I.

EXAMPLE III

This example illustrates the advantageous low extractability of variousbisurea UV stabilizers of the invention made with R groups (of FormulaI) derived from different organic diisocyanates.

The procedure of Example I for preparing solutions of spandex polymerwas repeated except that the antioxidant, the dye enhancer and thepolyol additives were omitted. UV stabilizers were added to the polymersolutions to form the following samples. Unless noted otherwise, theconcentration of UV stabilizer was 1.0% based on the weight of polymerin solution:

Sample III-1 bisurea UV stabilizer prepared as in Example I.

Sample III-2 bisurea prepared as in Ex. I, except the MDI was replacedby 44.3 grams of PICM.

Sample III-3 bisurea prepared as in Ex. I, except the MDI was replacedby 37.6 grams of IPDI.

Sample III-4 bisurea prepared as in Ex. I, except the MDI was replacedby 29.4 grams of TDI.

Comparison C "Tinuvin" 213.

Comparison D no UV-light stabilizer added.

Films were cast from the above-described solutions onto "Mylar"polyester sheets. A doctor knife with a 0.25-mm clearance was used. Thecast film was placed in an oven heated to 70° C. for about 16 hours topermit the solvent to evaporate from the film. The films were thenexposed in perchloroethylene in accordance with the extractability testdescribed hereinbefore. The following table summarizes the results ofthe test:

                  TABLE III                                                       ______________________________________                                                      % Extractability                                                ______________________________________                                        Sample of Invention                                                           III-1           10                                                            III-2           10                                                            III-3           <5                                                            III-4           10                                                            Comparison Sample                                                             C               80                                                            D               --                                                            ______________________________________                                    

The data of Table III on the extractability of UV stabilizer bysolvents, such as perchloroethylene, clearly show the vast superiorityof the bisurea stabilizers of the invention over the known commercialstabilizer (Comparison C) from which the bisureas of samples III-1through III-4 of the invention were derived. In these tests, 80% of thecommercial UV stabilizer ("Tinuvin" of Comparison Sample C) wasextracted from the film by the perchloroethylene. In contrast, 10% orless of the bisurea UV stabilizers of the invention was extracted by thesolvent.

I claim:
 1. An ultraviolet-light stabilizer for organic polymers, thestabilizer being a bisurea compound of the structural formula (I)##STR6## wherein R denotes a divalent group which remains after removalof the two isocyanate groups of an organic diisocyanate and is selectedfrom the group consisting of bis(p-phenylene)methane,bis(4cyclohexylene)methane, 3,3,5-trimethyl-5-methylene-cyclohexyl andtolylene,R¹ is a saturated hydrocarbon chain of 2 or 3 carbon atoms andR² is a3-(2H-benzotriazol-2-yl)-4-hydroxy-5-(1,1-dimethyl)benzenepropanoylgroup.
 2. A stabilizer in accordance with claim 1 wherein R isbis(p-phenylene)methane and R¹ is --CH2--CH2--.